1. Field of the Invention
The present invention generally relates to wireless communication systems and, more particularly, to the application of relays serving as an intermediate node for signal transmissions between user equipments and base stations.
2. Description of the Art
A communication system consists of a DownLink (DL), conveying transmissions of signals from a base station (Node B) to User Equipments (UEs), and of an UpLink (UL), conveying transmissions of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, etc. A Relay Node (RN) is an intermediate node between the Node B and the UE, it may participate in either or both the DL and UL transmission of signals, and it may be fixed or mobile.
Relay technology has been widely used to extend coverage in heavily shadowed areas in a cell or in areas beyond the Node B range. Moreover, although not yet extensively used for this purpose, RNs can provide gains in spectral efficiency particularly for UEs at a cell edge. Therefore, a RN may be used in a rural area to improve cell coverage, in an urban hot spot to increase spectral efficiency, or in a heavily shadowed area to avoid holes in coverage. Based on the protocol layer at which a data packet is available at the RN, the RN can be classified as Layer 0 (L0), Layer 1 (L1), Layer 2 (L2), or Layer 3 (L3).
An L0 RN is a Radio Frequency (RF) repeater operating at the PHYsical (PHY) layer. An L0 RN amplifies and forwards the received signal in the analog front-end for coverage extension. Since a L0 RN cannot distinguish the desired signal from interference and noise, it typically does not improve spectral efficiency. Also, RF repeaters require large transmitter/receiver isolation which requires a large device size and relatively high hardware and installation costs.
An L1 RN is also a repeater operating at the PHY layer which, after some base-band processing such as, for example, frequency domain filtering, amplifies only a portion of the received signal waveform. An L1 RN can provide only limited gains in the quality of the desired signal and is therefore not appropriate for improving spectral efficiency.
An L2 RN incorporates protocol layers above the PHY layer, such as the Medium Access Control (MAC) layer and possibly the Radio Link Control (RLC) layer, but it does not incorporate all protocol layers of a conventional Node B. For example, an L2 RN does not incorporate the Packet Data Convergence Protocol (PDCP) and Internet Protocol (IP) layers. L2 RNs can be further classified depending on their level of functionality. Several possible functionalities of L2 RNs exist. To solve the problems existing in the prior art, the present invention focuses on improving the following two functionalities:                a) The end-to-end operating points of the Hybrid Automatic Repeat reQuest (HARQ) process are between the Node B and the UE while the RN assists by transmitting data information, and possibly control information, to the Node B in the UL of the communication system or to the UE in the DL of the communication system.        b) The RN has available the MAC Protocol Data Units (PDUs). HARQ operates independently at the Node B and RN or at the UE and RN. The RN can perform its own scheduling and link adaptation. Link adaptation refers to the selection of the Modulation and Coding Scheme (MCS) and/or of the signal transmission power. Alternatively, the Node B can perform scheduling and link adaptation on behalf of the RN and signal the corresponding information to the RN.        
An L3 RN has all the functionalities of a Node B and therefore supports the whole IP/PDCP/MAC/PHY protocol stack. The L3 RN has its own Physical Cell Identity (PCI) and is typically indistinguishable to UEs from a regular Node B. IP packets are transported to the L3 RN on the relay backhaul link (between the relay and serving eNB).
A structure for a communication system incorporating an RN is illustrated in FIG. 1. The Node B 110 transmits and receives data or control signals 170D and 170U to and from UE1 120, respectively, through a direct link, regardless of the RN presence. The Node B 110 also transmits and receives data or control signals 160D and 160U to and from the RN 130, respectively, through a (wireless) backhaul link. The RN 130 transmits and receives data or control signals 150D and 150U to and from UE2 140, respectively, through an access link. The Node B 110 may or may not transmit or receive data or control signals 160D and 160U for UE2 140.
The link between the Node B and the RN (backhaul link) may be at the same or at a different frequency band than the link between the RN and the UE. If it is at the same frequency band, the RN is referred to as in-band; otherwise, it is referred to as out-band. In-band RNs need RF isolation because otherwise some of the signal from the RN transmitter will leak into the RN receiver which will cause positive feedback leading to operational failure. The present invention assumes that the RN transmits and receives signals in-band using Time Division Multiplexing (TDM). A guard time period exists between RN transmission and RN reception in order to switch between the transmitter RF and the receiver RF. Typically, RNs are assumed to operate in a half-duplex mode where the RN does not transmit and receive at the same frequency and at the same time.
FIG. 2 illustrates a framework for communication of a RN 210 with a Node B 220 and with UEs 230. The Transmission Time Interval (TTI) is assumed to be one sub-frame 240 which consists of transmission symbols. As the RN is assumed to not be able to simultaneously transmit and receive at the same frequency band, it can only transmit signals to UEs (access link) or to the Node B (backhaul link), or receive signals from the UEs (access link) or from the Node B (backhaul link). Five sub-frames 250T, 260T, 270T, 280T, and 290T, over a period of 10 sub-frames constituting one frame, are considered in FIG. 2 for RN transmission and live sub-frames 250R, 260R, 270R, 280R, and 290R, are considered for RN reception. Moreover, in some sub-frame symbols, the RN may be transmitting while in other sub-frame symbols the RN may be receiving. A guard period is obtained by partial or full puncturing of symbols at the beginning of the sub-frame, or at the end of the sub-frame, or both.
Link adaptation of DL transmissions is enabled by the Node B transmitting Reference Signals (RS) the UEs can use to derive a DL metric, such as the channel medium response or the Signal-to-Noise and Interference Ratio (SINR), at sub-bands of the operating BandWidth (BW). A UE can provide the Node B a Channel State Information (CSI), such as the SINR over sub-bands of the operating BW, through feedback in the UL thereby enabling link adaptation by the Node B for a subsequent DL transmission to the UE. Such an RS will be referred to CSI-RS. The CSI-RS is scrambled with a sequence derived from the PCI of the Node B. The Node B can inform the UE of the selected MCS through a control channel.
FIG. 3 illustrates a DL sub-frame structure assuming Orthogonal Frequency Division Multiplexing (OFDM) for the signal transmission method. The DL sub-frame consists of fourteen OFDM symbols 310. Some OFDM symbols are used to transmit the Physical Downlink Control CHannel (PDCCH) 320 conveying control information to UEs while the remaining OFDM symbols are used to transmit the Physical Downlink Shared CHannel (PDSCH) 330 conveying multiple Transport Blocks (TBs) of data information to multiple UEs, respectively. The number of PDCCH OFDM symbols may vary per sub-frame, for example between one and three. The PDCCH includes Scheduling Assignments (SAs) for DL or UL transmissions of TBs or acknowledgement signals as they are subsequently described. The Node B transmits a Physical Control Format Indicator CHannel (PCFICH) at predetermined locations in the first OFDM symbol (not shown for brevity) to inform the UEs of the PDCCH size. Each OFDM symbol is transmitted over an operating BW which consists of frequency resource units which will be referred to as Physical Resource Blocks (PRBs). Each PRB further consists of NwRB sub-carriers which are also referred to as Resource Elements (REs) 340. A UE is allocated MPDSCH PRBs for a total of MwPDSCH=MPDSCH·NwRB REs for PDSCH reception. FIG. 3 assumes a Node B with four transmitter antennas. The Node B transmits RS, RS1 350A and RS2 350B, from two antennas (for example, by combining pairs of the four transmitter antennas). These RSs can assist, for example, with PDCCH demodulation and will be referred to as Cell-specific RS (CRS). PDSCH demodulation can be based either on the CRS or on UE-specific Demodulation RS (URS) which is not shown for brevity. The Node B also transmits CSI-RS from each physical antenna, CSI-RS 1 360A, CSI-RS 2 360B, CSI-RS 3 360C, and CSI-RS 4 360D.
Link adaptation of UL transmissions is enabled by UEs transmitting RSs sounding a part or all the UL operating BW. Such RSs will be referred to as Sounding RSs (SRS). The Node B can then directly obtain the CSI for a UE and signal the MCS and/or power for UL signal transmissions through SAs in the PDCCH.
FIG. 4 illustrates an exemplary UL sub-frame structure for the transmission of a Physical Uplink Shared CHannel (PUSCH) using Single-Carrier Frequency Division Multiple Access (SC-FDMA). The PUSCH conveys TBs of data information and possibly control information from multiple UEs to the Node B. The UL sub-frame consists of fourteen SC-FDMA symbols 410 used to transmit data signals, control signals 420, or RSs 430 assisting in their Demodulation (DRS). The DRS can be based on the transmission of a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence having a Cyclic Shift (CS). The UL transmission BW also consists of PRBs with each PRB having NwRB REs. A UE is allocated MPDSCH PRBs for a total of MwPDSCH=MPDSCH·NwRB REs for its PUSCH transmission BW 440. The last sub-frame symbol may be used for SRS transmission 450, from one or more UEs. A similar sub-frame structure can be used for the Physical Uplink Control CHannel (PUCCH), which conveys Uplink Control Information (UCI) to the Node B. UCI includes HARQ ACKnowledgement (HARQ-ACK) signals, in response to PDSCH receptions, or CSI signals.
A UL SA is described through a set of contents in Table 1. Additional IEs or different number of bits for the indicative IEs in Table 1 may apply.
TABLE 1Information Elements of a UL SA for PUSCH Transmission.InformationNumberElementof BitsCommentPRB Allocation11Assignment of Consecutive PRBsMCS5MCS Levels (or Transport Block Size)NDI1New Data Indicator (synchronous HARQ)TPC Command2Power control commandsCS Indicator3Maximum of 8 CSHopping Flag1Frequency Hopping (Yes/No)CSI Request1Include CQI report (Yes/No)CRC (UE ID)16UE ID masked in the CRCTOTAL40
The first IE in provides the PUSCH PRB allocation. For an operating BW of P PRBs, the number of possible contiguous PRB allocations (for SC-FDMA) is 1+2+ . . . +P=P(P+1)/2 and can be signaled with ┌log2(P(P+1)/2)┐ bits where ┌ ┐ is the ceiling operation rounding a number to its next higher integer. For P=50 PRBs assumed in Table 1, the number of required bits is 11. The second IE provides the MCS. With 5 bits, up to 32 MCS can be indicated. For example, the modulation may be QPSK, QAM16, or QAM64 while the coding rate may take discrete values between 1/16 and 1. The third IE is the New Data Indicator (NDI). The NDI is set to 1 if the UE should transmit a new TB, while it is set to 0 if the UE should transmit the same TB as in a previous PUSCH transmission. Synchronous HARQ is assumed. The fourth IE provides the Transmission Power Control (TPC) command for the UE to adjust its PUSCH transmission power. For example, the 2 bits of the TPC IE, [00, 01, 10, 11], may respectively adjust the PUSCH transmission power by [−1, 0, 1, 3] deciBels (dBs). The fifth IE provides the CS of the CAZAC sequence serving as DRS. The sixth IE indicates whether frequency hopping applies to the PUSCH transmission. The seventh IE indicates whether the UE should include a DL CSI report in the PUSCH transmission. Each SA also contains a Cyclic Redundancy Check (CRC) which is typically scrambled with the UE IDentity (UE-ID).
A DL SA is described through a set of contents in Table 2. Additional IEs or different number of bits for the indicative IEs in Table 2 may apply.
TABLE 2Fields of a DL SA for PDSCH Transmission.InformationNumberFieldof BitsCommentResource Allocation1Type 0 or Type 1HeaderPRB Allocation25Assignment of RBsMCS or5MCS Levels orTB Size (TBS)Transport Block SizeHARQ Process3Up to 8 HARQ processesRedundancy2Up to 4 RVsVersion (RV)NDI1NDI (synchronous HARQ)TPC Command2Power control commandsCRC (UE ID)16UE ID masked in the CRCTOTAL55
The first IE is the Resource Allocation (RA) header and specifies the RA type. This is not material to the present invention, and for brevity, it is not further discussed. The second IE provides the PDSCH PRB allocation for the RA type. The third IE provides the MCS or the TB Size (TBS). The fourth IE indicates the HARQ process number (asynchronous HARQ is assumed) for 8 HARQ processes. The fifth IE indicates the Redundancy Version (RV) for the HARQ process assuming Incremental Redundancy (IR) for HARQ retransmissions of a TB. Four RVs are assumed, RV0, RV1, RV2, and RV3, and, in order to maximize the coding gains, they are used respectively for the initial TB transmission and for the first, second, and third TB retransmissions (maximum of 4 HARQ transmissions for a TB). The sixth IE is the NDI which is set to 1 if a new TB is transmitted and is set to 0 if the same TB, possibly with a different RV, is transmitted. The seventh IE provides the TPC command for power adjustments of the HARQ-ACK signal the UE transmits in the PUCCH in response to the PDSCH reception.
Another important classification of relays is whether they are transparent or non-transparent to UEs. A Transparent RN (T-RN) is indistinguishable to UEs from their serving Node B. This implies that a T-RN shares the same PCI as its associated Node B (L0/L1/L2 RNs). A Non-Transparent RN (NT-RN) appears to UEs as a separate Node B having its own PCI (e.g. L2/L3 RNs). The present invention considers transparent RNs. It is further assumed that a T-RN forwards only data signals from and to UEs (it does not forward control signals) and is therefore primarily beneficial for spectral efficiency gains and not for coverage extension (control signaling is assumed to be between a UE and the Node B).
The placement of an RN is typically such that it achieves a good (wireless) link quality to the Node B. A T-RN monitors the signals between the Node B and targeted UEs and attempts to decode the respective PDSCH or PUSCH transmissions. If the decoding at the T-RN is successful and the decoding at the UE (or Node B) is not, the T-RN will contribute through subsequent concurrent HARQ retransmissions in synchronized time/frequency resources. For example, in the DL, the Node B transmits the initial data packet to a UE. The transparent RN also receives this data packet. If the reception at the UE fails, as indicated by the subsequent transmission by the UE of a HARQ-ACKnowledgement (HARQ-ACK) signal with negative value (NACK), the T-RN can participate in the HARQ retransmission (after decoding and re-encoding the received data packet). This T-RN type is also known as a “Decode and Forward” RN. The Node B may or may not participate in the HARQ retransmission.
T-RNs are associated with a series of advantages such as the following:                a) T-RNs avoid frequent handovers and coverage imbalances associated with NT-RNs that create their own cells and typically have lower transmission power than the Node B. The coverage imbalance problem may often be so severe that additional measures, such as interference co-ordination, are required in order to enable sufficiently reliable NT-RN operation.        b) As the backhaul and access links of NT-RNs are TDM, an NT-RN serving many UEs requires extensive packet aggregation to align each UE's traffic in the same backhaul link sub-frame, thereby resulting in scheduling loss. T-RNs can, to a large extent, avoid this drawback of NT-RNs.        c) T-RNs may not need to waste resources to support a backhaul link and an access link, thereby directly avoiding a respective spectral efficiency loss.        d) T-RNs do not require the whole protocol stack, thereby offering lower implementation cost.        e) For synchronous, non-adaptive HARQ. T-RNs offer smaller latency as a backhaul link is not needed. In non-adaptive HARQ, the MCS and PRBs for signal transmission are the same as for the initial transmission.        
T-RNs are also associated with a series of implementation challenges:                a) T-RNs do not transmit their own CRS (as they do not transmit PDCCH) and consequently cannot support link adaptation for PDSCH transmissions to UEs. The PDSCH reception at the UE is assumed to be based on URS.        b) As a UE may insert UCI in its PUSCH transmission which the T-RN is aware of but cannot know its content in advance, it cannot forward such PUSCH transmissions to the Node B. This is because as the DRS transmitted by the T-RN overlaps with the DRS transmitted by the UE, the channel estimate obtained for the demodulation of the UCI which is transmitted only by the UE will not be accurate.        c) As the PDCCH size may vary per sub-frame, the T-RN needs to decode the PCFICH and to switch between reception mode and transmission mode before it transmits PDSCH. A simple approach solving this issue is to always have the maximum PDCCH size in sub-frames with RN PDSCH transmission. Then, the PDSCH size is also deterministic. This will not create any meaningful inefficiency as UEs served by the T-RN typically experience low SINR for Node B transmissions and consequently require a large PDCCH size when scheduled.        d) T-RN cannot support asynchronous or adaptive HARQ as the T-RN needs to know the scheduling information in advance in order to participate in a PDSCH or PUSCH transmission.        
Therefore, there is a need to enable link adaptation for T-RNs.
There is also a need to identify UEs for which the T-RN assists in the communication process with the Node B.
There is also a need to support PUSCH transmissions by T-RNs when a UE also includes control information in its PUSCH transmission.
Finally, there is also a need to enable synchronized DL HARQ transmissions between the Node B and the T-RN and synchronized UL HARQ transmissions between a UE and the T-RN.